Transmission line transformer



Dec. 28, 1965 A. LUNA 3,226,665

TRANSMISSION LINE TRANSFORMER Filed Aug. 1. 1961 INVENTOR PEI LR AR T" ATTY.

United States Pate 0,

. 3,226,665 TRANSMISSION LINE TRANSFORMER Agostino Luna, Milan, Italy, assignor to Marelli Lenkurt S.p.A., Milan, Italy Filed Aug. 1, 1961, Ser.- No. 1 28,479 Claims priority, application Italy, Aug. 3, 1960, 13,869/ 60 3 Claims. (CL 336-182) This invention relates to transmission line transformers, that is, transformers for broadband transmission of signals. As it is known, a particular characteristic of these transformers is that they act in such a way to eliminate the cut-off effect in the high frequencies; this effect appears in conventional transformers because of the leakage inductances owing to magnetic fluxes that are generated in a winding and are not coupled to the other Winding, or because of the stray capacities existing among the various turns and forming some paths through which the signal current leaks.

The stucture of these transmission line transformers is Obtained by placing the primary winding wire beside the secondary winding wire and by winding the bifilar line so obtained around a core. In this way, the leakage inductance and the stray capacities are distributed uniformly along the two conductors and the cut-off effect cannot appear, since the said conductors form in effect a transmission line.

In the transformers so realized the two windings have necessarily the same number of turns and it is possible to obtain only the transformation ratios 1:1 and 1:2 by employing the two-windings separately as a primary and secondary, or by connecting them as an auto-transformer.

This limitation is very important for all the cases wherein the transformer shall be usedas an vimpedance transformer, as for instance, in the interstagef circuits of transistor 100 or 200 me. band amplifiers; I

'An object .of this invention is to obviate the aforesaid limitations. by realizing a transmission line transformer which permits any rational transformation ratio (ratio of two whole numbers) and therefore a gain greater than the one obtainable at present.

The transmission line transformer according to this invention is characterized in that it comprises more bifilar line sections wound on the same core and having the same number of turns and connected so that the input voltage of any line section whatsoever may beadded in phase to the output voltage of the same section, and the output voltage of each line section may coincide with the input voltage of the next line section. I

The number of sections is equal to 12-1, where n is representing the preferred. maximum transformation ratio. It is possible to obtain from the transformer so realized all transformation ratios rzs, where rand s are whole num- 'bers comprised between 1 and n.

This invention, by way. of example, will now be described and illustrated with reference to the enclosed drawings which represent schematically the bifilar line sections of a transformer and a diagram of connection.

FIG. 1 shows a distributed constants transformer having a ratio 1 :2 in transmission line form.

FIG. 2 shows a transmission line form of a distributed constants transformer having a maximum transformation ratio of 1:3.

FIG. 3 shows a transmission line form of a distributed constants transformer having a maximum transformation ratio of 1:5.

FIG. 4 shows a wiring diagram of the transformer which is shown in transmission line form in FIG. 3.

In FIG. 1, there is shown the transformer having a ratio 1:2, in the form of a transmission line. This figure there is the voltage 5E I 3,226,665 Patented Dec. 28, 1965 is a symbolic representation of the prior art arrangement shown in FIGI 1 'of Ruthroif Patent 3,03 7,175.

The single section of the bifilar line comprises two wires placed side by side, one extending from point 1 to point 3, the "other from point'2 to point 4, to form the two windings, and points 2' and 3 are connected together. The voltage E applied to the terminals 1-2 appears doubled between theterminals 1-4, since to the voltage E is added the voltage E existing between the terminals 3-4, the latter voltage being equal to E which is propagated along the line.

It will be apparent from FIG. 2 that if we want to obtain a transformation ratio 1:3, that-is, a voltage three times greater than E it is sufiicient to add to the line section of FIG. 1 a-second section, wherein the added section extends from points 5-6, to points 7-8, with points 5-6 connected respectively to points 3-4, and with point '7 connected to point 6.

If a transformer so realized is fed with a voltage E between the points 1-2, and if the total length of the two line sections (1-2)+ (3-4) and (5-6)+ (7-8) is very small with respect to the wave length of the applied voltage E and furthermore, the load resistance is such as 'to give no reflections to the terminals 7-8, the voltage between the points 1-8 will be equal to three times E.

In the case of a transformer having a transformation ratio 1:5, the number of line sections is four and they are connected as shown in FIG. 3; that isthe first section extends from points 1-2 (input) to points 3-4; the'second section extends from points 5-6 to points 7-8; the third section'extends from points 9-10, which are respectively connected to points 7-8, to points 1-1-12;'the fourth sec- -tion extends from points 13-14, which are respectively corresponding to 11-12, to points 15-16, where 15 is connected with 14. i

Of course, if moreline sections are added, it is possible to obtain any transformation ratio 151;, where n is 'a whole number.

From-the'above. it may-be deduced that in order to realize a transformer having a ratio 1:n, it isnecessary to employ n-1 bifilar windings.

Furthermore with'reference to FIG. 3, we note that, when between points -1 and 2, the voltage E is present and the whole winding consisting of the four sections is short with respect to the wave length of E and the transformer is matched, the following voltages exist: between points 1-4-there is the voltage 2E between points 1-8 there is the voltage 3E between points 1-12 there is the voltage 4E and finally between point 1 and point 16 If instead of applying the signal between 1 and 2 and drawing it between 1 and 16, the signal is applied between 1 and 4 or between 1 and 8 or between land 12, drawing it always between 1 and 16, it is possible to realize the transformation ratios 2:5 or 3:5 and 4:5 respectively.

Therefore we may deduce that a transformer, formed by n1 windings of bifilar sections, permits to realize, besides the transformation ratio 1:n, also all fractional ratios 2:11, 3m, 4:n, (n1):n comprised between 1m and mu.

On the other hand, if the signal is applied between 1 and 3, drawing it between 1 and 8, or if thesignal is applied between these two points, drawing it between 1 and 12, then the ratios 2:3 and 3:4 areobtained.

More in general, therefore, we may say that, if n1 is the number of bifilar line sections of the transformer, it is possible to obtain r:s transformation ratios, where r and s are whole numbers, comprised between 1 and n. Since, as is well known, there is a phase shift in a length o-f line equal to one half wave length as propagated along the line, for the voltages to add substantially in 3 phase it is evident that the total length of all of the sections must be substantially less than one half wave length at the maximum frequency in the desired range.

The wiring of the transmission line transformer of FIG. 3 is shown in FIG. 4, wherein, for a better understanding, the core N is supposed to be indefinitely rectilinear.

The connections between the four bifilar line sections are deduced from the same FIG. 3.

In fact a connection joins together the terminals 23-5; a Second connection joins together the points 467-9; and a third connection joins together the points 8-10- 11-13, etc., that is, the terminals of FIG. 3 which flow together into a single node, are connected.

The practical method to arrange these connections is as follows: after having established a progression in order to number the line sections (or partial windings) and its terminals, the second terminal of the first wire of a given line section is connected with the first terminal of the second wire of the same section and with the first terminal of the first wire of the next section, while the second terminal of the second wire of the said given section is connected with the first terminal of the second wire of the next section.

Considering now only the first two sections, the above means that we have to connect the output 3 of the first wire of the first section with the input 2 of the second wire of the same section and furthermore to connect the output 3-4 of the first section with the input -6 of the next section.

In FIG. 4, for a better understanding, the two wires of each Winding or bifilar section are drawn in a different way (thick lines for the first and thin lines for the second one), but in reality, in the single bifilar couples, a Wire is completely equivalent to the other one, and it is possible to determine arbitrarily which is the first and which is the second.

Consequently in the same way as for the connection of FIG. 3, when a voltage E is applied to the input terminals 1-2 of the transformer of FIG. 4, it is possible to draw a voltage 2E between the output terminals 1-4; a voltage 3E between 1-8; a voltage 4E between 1 12 and a voltage 5E between 1-16.

What is claimed is:

1. In combination, a two-element transmission line comprising a plurality of sections, each section comprising a first wire and a second wire wound together, said wires being contiguous and parallel from a first end of eachto a second end of each to form a pair of coils, said coils being serially connected with the second end of the first wire connected to the first end of the second wire; said sections being connected in cascade with the second ends of said first and second wires of each section except the last connected to the first ends of the corresponding wires of the next following section; connections for applying an input between the first end of the first wire of the first sectionand second end of one of the wires of one of the sections, and-connections for an output between the first end of the first wire of the first secwithinsaid range, so that when impedance matched at said input and said output the voltages of all of said sections are substantially in phase and so that with n-l line sections the possible transformation ratios are given by the ratio rzs, where r and s are whole numbers comprised between 1 and n inclusive.

3. A broadband transformer comprising a plurality of line sections;

each section comprising a two-element transmission line comprising a first wire and a second wire wound together between first and second terminal points as a pair at one end and third and fourth terminal points as a pair at the other end with the first wire wound between the first and third terminal points and the second wire wound between the second and fourth terminal points, said wires being contiguous from said one end to said other end to form a pair of coils, a connection from the second terminal point to the third terminal point placing said coils of the section in series, placing said coil formed by the first wire across said first and second terminal points and placing said coil formed by the second wire across said third and fourth terminal points; said sections being connected in cascade with the third and fourth terminal points of each section except the last connected respectively to the first and second terminal points of the next following section so that the coil formed by the second wire of each section is connected in parallel with the coil formed by the first wire of the following section, the said sections having the same number of turns; connection for an input circuit being provided between two of said terminal points, and connection for an output circuit being provided between two of said terminal points;

the total length of said transmission line comprising all of said sections being substantially less than a half wave length at the maximum frequency of operation so that the voltage across the first and second terminal points of any section is added substantially in phase with the voltage across the third and fourth terminal points of the same section and the voltage across the third and fourth terminal points of each section except the last is applied as the voltage across the first and second terminal points of the succeeding section, and the voltages across said terminal-point pairs of all the sections are substantially of the same value and in phase with one another, so that with n1 line sections the possible transformation ratios are given by the ratio rzs where r and s are whole numbers comprised between 1 and n inclusive,

References Cited by the Examiner UNITED STATES PATENTS 1,762,775 6/1930 Ganz 336181 X 2,550,891 5/1951 Wall 336-205 2,654,836 10/1953 Beck et a1. 33325 3,037,175 5/1962 Ruth-off 33332 3,114,120 12/1963 Heck 336-189 X FOREIGN PATENTS 665,834 9/ 1938 Germany,

JOHN F. BURNS, Primary Examiner. LARAMIE E. ASKIN, Examiner. 

1. IN COMBINATION, A TWO-ELEMENT TRANSMISSION LINE COMPRISING A PLURALITY OF SECTIONS, EACH SECTION COMPRISING A FIRST WIRE AND A SECOND WIRE WOUND TOGETHER, SAID WIRES BEING CONTIGUOUS AND PARALLEL FROM A FIRST END OF EACH TO A SECOND END OF EACH TO FORM A PAIR OF COILS, SAID COILS BEING SERIALLY CONNECTED WITH THE SECOND END OF THE FIRST WIRE CONNECTED TO THE FIRST END OF THE SECOND WIRE; SAID SECTIONS BEING CONNECTED TIN CASCADE WITH THE SECOND ENDS OF SAID FIRST AND SECOND WIRES OF EACH SECTION EXCEPT THE LAST CONNECTED TO THE FIRST ENDS OF THE CORRESPONDING WIRES OF THE NEXT FOLLOWING SECTION; COMNECTIONS FOR APPLYING AN INPUT BETWEEN THE FIRST END OF THE FIRST WIRE OF THE FIRST SECTION AND SECOND END OF ONE OF THE WIRES OF ONE OF THE SECTIONS, AND CONNECTIONS FOR AN OUTPUT BETWEEN THE FIRST END OF THE FIRST WIRE OF THE FIRST SECTION AND THE SECOND END OF THE OF THE WIRES OF ONE OF THE SECTIONS. 